package leetcode.editor.cn;
//给你一个单链表的头节点 head ，请你判断该链表是否为回文链表。如果是，返回 true ；否则，返回 false 。 
//
// 
//
// 示例 1： 
//
// 
//输入：head = [1,2,2,1]
//输出：true
// 
//
// 示例 2： 
//
// 
//输入：head = [1,2]
//输出：false
// 
//
// 
//
// 提示： 
//
// 
// 链表中节点数目在范围[1, 105] 内 
// 0 <= Node.val <= 9 
// 
//
// 
//
// 进阶：你能否用 O(n) 时间复杂度和 O(1) 空间复杂度解决此题？ 
// Related Topics 栈 递归 链表 双指针 
// 👍 1639 👎 0


//leetcode submit region begin(Prohibit modification and deletion)

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;

/**
 * Definition for singly-linked list.
 * public class ListNode {
 * int val;
 * ListNode next;
 * ListNode() {}
 * ListNode(int val) { this.val = val; }
 * ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution42 {
    public boolean isPalindrome(ListNode head) {
        ListNode dummy = new ListNode(-1);
        ListNode p = head;
        while (p != null) {
            ListNode next = new ListNode(p.val);
            next.next = dummy.next;
            dummy.next = next;
            p = p.next;
        }

        ListNode q = dummy.next;
        while (head != null) {
            if (head.val != q.val) return false;
            head = head.next;
            q = q.next;
        }
        return true;
    }


    public boolean isPalindrome1(ListNode head) {
        Stack<Integer> stack = new Stack<>();
        int size = 0;
        while (head != null) {
            size++;
            stack.push(head.val);
            head = head.next;
        }


        int num = 0;
        List<Integer> list = new ArrayList<>();
        while (!stack.isEmpty()) {
            if (size / 2 > num) {
                num++;
                list.add(stack.pop());
            } else {
                if (size % 2 != 0) {
                    stack.pop();
                }
                break;
            }
        }
        num = size / 2 - 1;
        while (!stack.isEmpty()) {
            if (list.get(num) == stack.peek()) {
                num--;
                stack.pop();
            } else {
                return false;
            }
        }
        return true;

    }

    public class ListNode {
        int val;
        ListNode next;

        ListNode() {
        }

        ListNode(int val) {
            this.val = val;
        }

        ListNode(int val, ListNode next) {
            this.val = val;
            this.next = next;
        }
    }
}
//leetcode submit region end(Prohibit modification and deletion)
